Semiconductor manufacturing consists of a number of crucial processing steps performed on wafer lots where measurements of minimum feature sizes known as critical dimensions (CD) are made to ensure proper device fabrication. The high-degree of precision during this processing requires the utilization of Scanning Transmission Electron Microscopes and Transmission Electron Microscopes (S/TEM) to calibrate in line CD-SEM instruments. These critical tools provide measurement capabilities in the low nanometer range. Accuracy of their measurements is essential since effective process controls depend on CDs they supply. S/TEMs require frequent calibration to ensure their accuracy since processing errors cause appreciable variation in CDs. Calibration procedures are very time consuming and have a negative impact on semiconductor fabrication workflow. Typical S/TEM magnification calibration approaches are also limited in accuracy.
A conventional calibration approach utilizes a standard that possesses a crystal lattice specimen to calibrate a S/TEM for a particular relatively high magnification. Measurements obtained from the specimen's crystal lattice spacing are compared to known data to determine if the S/TEM requires adjustment, or if the magnification results being calibrated are within tolerance. Calibration adjustments are then made accordingly. This approach requires that calibration for other magnifications in the range of magnifications, such as relatively lower magnifications, required to support semiconductor fabrication use different standards. Thus, multiple specimen exchanges along with new orientations to achieve calibration over a range of magnifications are required. In addition, the beam conditions must be reset for each standard. This iterative process is conducted until calibration is achieved for the full range of magnification required in support of semiconductor fabrication.
Another technique uses a calibration standard that includes one or two repeating features that are calibrated to a single dimensional value, such as a series of parallel lines spaced apart at a predetermined, calibrated distance. Such a standard is only useful over a certain portion of the usable magnification range of an S/TEM, thereby again requiring several different standards having appropriately varying sized features to allow the S/TEM to be calibrated over its useable range. Because S/TEM's are typically operating in a high vacuum environment, the vacuum must be broken and re-established each time a standard is changed for a different range of calibrations, resulting in an undesirable time delay. Furthermore, the use of repeating features such as uniformly spaced apart lines requires an operator calibrating the instrument to manually count the number of repeating features and cross reference the known feature dimensional values to arrive at a distance for the features to be used in calibrating the instrument. Accordingly, an operator may need to count as many as 50 repeating features, look up the dimensional value, and then multiply the number of features by the dimensional value to calculate a size measurement for calibration. This process may need to be repeated for multiple magnification values, typically 15 or more magnification values, using several different standards to cover the magnification range of the S/TEM. Such a process is time consuming and error prone.